Image sensors may be semiconductor devices that convert optical images into electrical signals. Complementary metal oxide semiconductor (CMOS) image sensors may use CMOS fabrication technology to create photosensitive devices that capture and process optical images within a single integrated chip. A photodiode may typically be used as a photodetector in the CMOS image sensors. CMOS image sensors may have advantages over traditional charge-coupled devices (CCDs). In particular, a CMOS image sensor may have a high image acquisition rate, lower operating voltage, lower power consumption, and higher noise immunity. In addition, CMOS image sensors may be fabricated on the same high-volume wafer processing lines as general logic and memory devices. As a result, a CMOS image chip may comprise both image sensors and all necessary logic devices, such as amplifiers, analog-to-digital converters, and the like.